Scalarization of Charged Black Hole in Gauss-Bonnet Extended Starobinsky-Maxwell Gravity

Abstract

We re-examine spontaneous scalarization of black holes within Starobinsky gravity supplemented by the Gauss-Bonnet invariant. A novel feature is uncovered: scalarized solutions split into two smooth branches that connect at a common minimal horizon radius, a multi-branch structure previously unreported for static, spherically symmetric scalarization. Extending the theory with a Maxwell field, we find that this multi-branch structure persists, while a new additional disconnected branch emerges within certain parameter ranges. We thoroughly analyse the transition from a single smooth branch to two disconnected branches. Lastly, we verify, employing the standard Maxwell thermodynamic relations, that all charged hairy solutions obey the first law exactly.